Two bugfixes to field handling during thread cloning

embed.fnc

@@ -6425,7 +6425,7 @@ Rdp	|PADLIST *|padlist_dup	|NN PADLIST *srcpad			\
 Rdp	|PADNAME *|padname_dup	|NN PADNAME *src			\
 				|NN CLONE_PARAMS *param
 Rdp	|PADNAMELIST *|padnamelist_dup					\
-				|NN PADNAMELIST *srcpad 		\
+				|NULLOK PADNAMELIST *srcpad		\
 				|NN CLONE_PARAMS *param
 Cp	|yy_parser *|parser_dup |NULLOK const yy_parser * const proto	\
 				|NN CLONE_PARAMS * const param

inline.h

@@ -4681,7 +4681,8 @@ Perl_padname_refcnt_inc(PADNAME *pn)
 PERL_STATIC_INLINE PADNAMELIST *
 Perl_padnamelist_refcnt_inc(PADNAMELIST *pnl)
 {
-    PadnamelistREFCNT(pnl)++;
+    if (pnl)
+        PadnamelistREFCNT(pnl)++;
     return pnl;
 }
 

pad.c

@@ -1295,6 +1295,7 @@ S_pad_findlex(pTHX_ const char *namepv, STRLEN namelen, U32 flags, const CV* cv,
         return NOT_IN_PAD;
 
     if (PadnameIsFIELD(*out_name)) {
+        assert(PadnameFIELDINFO(*out_name));
         HV *fieldstash = PadnameFIELDINFO(*out_name)->fieldstash;
 
         /* fields are only visible to the class that declared them */
@@ -2744,13 +2745,15 @@ Duplicates a pad name list.
 PADNAMELIST *
 Perl_padnamelist_dup(pTHX_ PADNAMELIST *srcpad, CLONE_PARAMS *param)
 {
-    PADNAMELIST *dstpad;
-    SSize_t max = PadnamelistMAX(srcpad);
-
     PERL_ARGS_ASSERT_PADNAMELIST_DUP;
 
+    if (!srcpad)
+        return NULL;
+
+    SSize_t max = PadnamelistMAX(srcpad);
+
     /* look for it in the table first */
-    dstpad = (PADNAMELIST *)ptr_table_fetch(PL_ptr_table, srcpad);
+    PADNAMELIST *dstpad = (PADNAMELIST *)ptr_table_fetch(PL_ptr_table, srcpad);
     if (dstpad)
         return dstpad;
 
@@ -2827,6 +2830,7 @@ Perl_newPADNAMEouter(PADNAME *outer)
     PadnameREFCNT_inc(PADNAME_FROM_PV(PadnamePV(outer)));
     PadnameFLAGS(pn) = PADNAMEf_OUTER;
     if(PadnameIsFIELD(outer)) {
+        assert(PadnameFIELDINFO(outer));
         PadnameFIELDINFO(pn) = PadnameFIELDINFO(outer);
         PadnameFIELDINFO(pn)->refcount++;
         PadnameFLAGS(pn) |= PADNAMEf_FIELD;
@@ -2849,6 +2853,7 @@ Perl_padname_free(pTHX_ PADNAME *pn)
         if (PadnameOUTER(pn))
             PadnameREFCNT_dec(PADNAME_FROM_PV(PadnamePV(pn)));
         if (PadnameIsFIELD(pn)) {
+            assert(PadnameFIELDINFO(pn));
             struct padname_fieldinfo *info = PadnameFIELDINFO(pn);
             if(!--info->refcount) {
                 SvREFCNT_dec(info->fieldstash);
@@ -2900,18 +2905,27 @@ Perl_padname_dup(pTHX_ PADNAME *src, CLONE_PARAMS *param)
     PadnameTYPE   (dst) = (HV *)sv_dup_inc((SV *)PadnameTYPE(src), param);
     PadnameOURSTASH(dst) = (HV *)sv_dup_inc((SV *)PadnameOURSTASH(src),
                                             param);
-    if(PadnameIsFIELD(src) && !PadnameOUTER(src)) {
+    if(PadnameIsFIELD(src)) {
+        assert(PadnameFIELDINFO(src));
         struct padname_fieldinfo *sinfo = PadnameFIELDINFO(src);
-        struct padname_fieldinfo *dinfo;
-        Newxz(dinfo, 1, struct padname_fieldinfo);
-
-        dinfo->refcount   = 1;
-        dinfo->fieldix    = sinfo->fieldix;
-        dinfo->fieldstash = hv_dup_inc(sinfo->fieldstash, param);
-        dinfo->paramname  = sv_dup_inc(sinfo->paramname, param);
-
-        PadnameFIELDINFO(dst) = dinfo;
+        struct padname_fieldinfo *dinfo = (struct padname_fieldinfo *)ptr_table_fetch(PL_ptr_table, src);
+        if (dinfo)
+            PadnameFIELDINFO(dst) = dinfo;
+        else {
+            Newxz(dinfo, 1, struct padname_fieldinfo);
+            PadnameFIELDINFO(dst) = dinfo;
+            ptr_table_store(PL_ptr_table, sinfo, dinfo);
+
+            /* We must have set PadnameFIELDINFO(dst) before we recurse into
+             * fieldstash in case it points back here */
+            dinfo->refcount   = 1;
+            dinfo->fieldix    = sinfo->fieldix;
+            dinfo->fieldstash = hv_dup_inc(sinfo->fieldstash, param);
+            dinfo->paramname  = sv_dup_inc(sinfo->paramname, param);
+        }
+        assert(PadnameFIELDINFO(dst));
     }
+
     dst->xpadn_low  = src->xpadn_low;
     dst->xpadn_high = src->xpadn_high;
     dst->xpadn_gen  = src->xpadn_gen;

t/class/threads.t

@@ -21,6 +21,10 @@ class Testcase1 {
     method x { return $x }
 }
 
+class WithNoFields {
+    # a class with no fields, in order to test [GH23771]
+}
+
 {
     my $ret = threads->create(sub {
         pass("Created dummy thread");